Telescopic shafts mentioned above are used as steering shafts for automobiles, for example.
Further, the telescopic shafts are used as intermediate shafts for connecting steering shafts for automobiles and steering mechanisms such as rack-and-pinion mechanisms. In this case, the telescopic function of the telescopic shaft is used for adjusting the length of the intermediate shaft for absorbing relative displacement between a steering gear and a steering column when a vehicle is traveling. Alternatively, the telescopic function is used for adjusting the length of the intermediate shaft when the intermediate shaft is assembled into the vehicle.
As this type of telescopic shaft, a telescopic shaft has been known that fits an inner shaft and a tubular outer shaft into each other with a plurality of rollable balls interposed between corresponding axial grooves of both the shafts.
It is considered to provide a circumferential stopper between the inner shaft and the outer shaft such that both the shafts can transmit torque to each other by circumferentially engaging with each other even when the ball comes off the axial groove.
For example, a telescopic shaft has been proposed whose inner and outer shafts are made polygonal in cross section (see Japanese Patent No. 2632892, for example).